Integrated circuits are often manufactured on a semiconductor substrate, such as a silicon wafer. The silicon wafer is typically a thin circular plate of silicon that is 150 or 200 millimeters in diameter and approximately 25 mils thick. A single wafer will have numerous devices which are integrated circuits and are imprinted on the wafer comprising a lattice of devices. Each device consists of numerous layers of circuitry and a collection of bonding pads. The bonding pads are small sites, typically 3 mils square, made usually with aluminum that eventually serve as the device's connections to the pin leads. Other than the bonding pads, the remainder of the wafer is coated with a final layer of an insulating material such as silicon nitride, called the passivation layer, which in many respects behaves like glass. The aluminum its;elf forms a thin non-conductive layer of aluminum oxide, which must be eliminated or broken through before good electrical contact can be made.
Since the packaging of a device is somewhat expensive, it is desirable to test a device before packaging to avoid packaging bad devices. This process of testing devices before packaging is referred to as the sort process. This process involves connecting a device called a probe card to a special tester. The probe card has a collection of electrical contacts or pins that stands in for the normal pins and wire leads of a packaged device. The wafer is then positioned so that the contacts or pins on the probe card make contact with a given device's bonding pads and the tester runs a battery of electrical tests on the device. A special machine, called a wafer prober, is used to position each device on the wafer with respect to the probe card. High accuracy is required, because the bonding pads are small and if a probe card pin makes contact outside the bonding pad area, the result may be a break in the passivation layer, which generally results in a damaged device.
The primary purpose of wafer probing is to accurately position the collection of devices, or dice, on a wafer in such a manner so that the device's bonding pads make good electrical contact with a probe card's probe tips so that the device may be properly tested before dicing and packaging. There are several different considerations involved in wafer probing. First, due to the thin layer of nonconductive aluminum oxide that forms over the bonding pad during normal atmospheric exposure, there is a requirement that the probe tips travel vertically beyond initial contact. Furthermore, in the most common form of probe card technology, cantilever probes, a portion of this vertical overtravel is transformed into motion along the plane of the wafer, or the scrub, to further guarantee that the tip is in good contact with aluminum and not the oxide. Second, the devices, with the exception of the bonding pads, are coated with an insulating layer. If this substance, which is Essentially glass, is violated by the probe tips, the resultant cracks may damage an otherwise functional device. Given that the typical travel of the probe tip may be 1.5 to 2 mils and that the pad dimensions, or the area where contact is allowed, range in size from 2 to 4 mils, the requirements of accuracy are patently important.
The wafer prober, or prober, is required to do several things in order to probe a wafer. First, the prober, when given a wafer, must be able to accurately align the axis of the indexed dice on the wafer to a specified angle relative to the motion axis of the prober motor. This is generally accomplished via auto alignment. As an additional aspect of auto alignment, the prober needs to be able to find some known position on a wafer relative to some position on the motor repeatably from wafer to wafer in order to avoid making every wafer a training wafer. Second, the prober must know some location of the motor which is known to satisfy the requirements of good probe to pad contact, and must be able to index accurately from that location to the other dice in such a fashion so that the good probe to pad contact is repeated on subsequent dice. The process of finding the location of the motor which yields good probe to pad contact is referred to as probe to pad alignment. An often us;ed synonym for probe to pad contact is setting the first die. In addition to this, the prober must also be taught the vertical contact height of the probes and be able to accurately compensate for varying wafer thickness which is accomplished via wafer thickness profiling. Finally, since not all probe cards can be rotated accurately, an allowance must be made for the prober to be told the probe card's angle, to align wafers to the angle and to additionally index along axes rotated at that angle.
A wafer prober might actually be considered a three dimensional positioner. In addition however, there are a wide variety of features that are involved in the probing process. For example, in order to indicate which dice fail test, the user may wish to ink the dice. These various features together with the means for the users to control them and to monitor the testing process further define the prober and separate it from a simple positioning stage.
Accuracy and throughput issues have led to the development of the notion of the automatic wafer prober. Automation requires that the prober understand the lattice of devices and understand its relationship to the wafer and to the probe card. Furthermore, the system is trained, or the probe card location is shown, only on one wafer per device type. Thus, the system is required to repeat accurate positioning on subsequent like wafers.
Of the three main processes required for good probing--wafer alignment, thickness profiling, and probe to pad alignment--only probe to pad alignment has not traditionally been automated. Previously, while cumbersome, there was not as strong a requirement for automation of probe to pad alignment as the automation of the other two processes. However, developments in semiconductor technology have driven a requirement for automation for several reasons. First, probe cards can now have well in excess of 500 pins. Second, probe array sizes have become larger so that multiple die arrays can now be several inches in one dimension. Third, the tip diameter to pad dimension ratio has become close to one and the pitch, or distance between pins, is approaching the pad dimension. Consequently, the ability of the operator to accurately and rapidly align probe to pads using a microscope becomes more difficult. Hence, there is a growing desire to automate probe to pad alignment.
All semiconductor manufacturer's have the requirement to align probe to pads and wafer sort. Their challenge is to perform this relatively mundane task in the shortest possible time with the greatest consistency of accuracy and alignment.
In addition to the general automation requirement certain probing technologies are evolving which make top side microscope access to the probe array impossible. Prior art membrane probe is a technology which places a plastic membrane directly in the microscopes line of site and makes traditional alignment impractical. Furthermore, prior art high density probe arrays also preclude top side microscope probe to pad alignment. To cope with these new probing technologies and also to continue the pursuit of full automation in the wafer sort area, semiconductor manufacturers require an automatic hands-off light-off probe to pad alignment capability.
Moreover, there are several additional factors that add to the difficulty of probe to pad alignment using a blank aluminized wafer. First, the probe tips at initial contact do not correspond to the pad centers. Second, probe arrays are often not composed of continuous lines; often there are offset pads missing sides or corners, double rows and many other exceptions. Third, dirt in the form of aluminum particles is often left behind on the wafers as a result of the manufacturing process. Fourth, the blank wafer is highly reflective which exacerbates bad lighting and dirty prober microscope optics.
Major vendors of wafer probers have offered some form of automatic probe to pad alignment. However, the forms of automatic probe to pad alignment offered are often not sufficient to provide alignment for modern integrated circuits which have very small, pads and probe tips which are also small and often have imperfect shapes. Consequently, there is a requirement to automatically determine the position coordinates of a probe array and the position coordinates of a first die with sufficient accuracy.